boards: st: add support for stm32mp257f_ev1 board

Introduce initial board support for the STM32MP257F-EV1. This includes:
- Board metadata (board.yml, stm32mp257f_ev1.yaml)
- Base software configuration (Kconfig.stm32mp257f_ev1)
- CMake build integration (board.cmake)
- Default configuration (stm32mp257f_ev1_stm32mp257fxx_m33_defconfig)
- Minimal device tree (stm32mp257f_ev1_stm32mp257fxx_m33.dts)
- Initial documentation and image for the stm32mp257f_ev1 board.
- OpenOCD configuration files for debugging support.

This enables the STM32MP257F-EV1 board to build and debug a hello world
sample.

Note that other necessary openocd configuration files are added to the
zephyr sdk directory and can be found in the stm-openocd repository[1].
[1]:https://github.com/STMicroelectronics/device-stm-openocd/tree/main



Signed-off-by: Youssef Zini <youssef.zini@savoirfairelinux.com>